Joseph Parker's stamina and conditioning will face their biggest test when he takes on American-based heavyweight Sherman Williams on the undercard of Alex Leapai's world title challenge in Germany next month.

Fairfax Media can reveal Parker (7-0) has secured a change in opponent for the high-profile April 26th card. The 22-year-old was originally scheduled to meet undefeated Ukrainian Andriy Rudenko (24-0). He will now take on Williams (36-13), one of the world's more durable heavyweights, over 10 rounds.

Parker's promoters were told Rudenko has a foot problem.

While Williams has lost three of this last six fights to credible opponents, which included a no-contest result after a headbutt incident with Evander Holyfield, the 41-year-old has been stopped only once, against Robert Davis in 1999, in his 17-year, 52-fight career.

Williams, who is sparing world champion Wladimir Klitschko in the lead-up to his challenge from Leapai, also defeated American-Kiwi Chauncy Welliver (55-8) with a majority decision in 2012.

"He's not in the class of Rudenko but he certainly serves as a very strong test for Joseph," Parker's trainer, Kevin Barry, said from their Las Vegas training base. "There are not too many guys around with 52 fights that have been stopped only once."

Bahamas-born Williams is a come-forward fighter who likes to close the distance and throw his over-hand right but, if Parker follows the plan to use his height and reach advantage coupled with his fast hands, the veteran could be in for a long night.

"If Joseph can knock Sherman out it's going to make noise globally. That will be the goal," Duco promoter David Higgins said. "We want to test Joseph. We're not looking to pad his record. We don't want to take any shortcuts."

Barry was comfortable with the late change in opponent, which leaves the pair just over a month to make adjustments.

Parker is into his seventh week of training and has already got through 44 rounds of sparing. While his power is no longer questioned, the South Auckland prospect is likely to be pushed past the sixth round for the first time in his fleeting pro career.

"We haven't had to change a lot," Barry said. "There are several combinations I think will work better on Williams, so we've added those in the gym.

"Joseph is in very good shape. I'm very happy with where he's at skill wise for a fighter who's only being a pro for a year-and-a-half. His stamina, fitness and strength are improving all the time.

"We will be conditioned and prepared to go 10 rounds. I'm very confident we will still be throwing a lot of punches if it goes that long."

A strong performance on the world stage could quickly open doors for Parker.

"There's a huge upside if Joseph boxes impressively here. He understands the significance, as a 7-0 prospect, to be fighting as one of the television broadcast fights on a heavyweight championship card. It's huge," Barry said. "That is the motivation to make sure he puts on an impressive display."